A transfer switch is an electromagnetic mechanism that controls to which power source a load is connected, such as between a utility (main) power source and a backup power source. An automatic transfer switch can include a DC motor that drives an attached mechanical device, which is typically coupled to the motor through an intermediary gearing system such as a gearbox.
As the motor is switching between power sources, eventually the mechanism will reach a point where it cannot be moved any further, requiring intervention to stop the motor from driving the mechanism. If the motor is continues to turn, the motor and gearbox can be damaged and cause the system to fail to operate. The motor can be damaged by twisting the motor shaft or loosening or breaking the motor's mounting to the device. The gearbox can be damaged by cracking the gears, breaking off gear teeth, or loosening or breaking the gearbox's mounting to the device.
A common solution to prevent the motor from driving the mechanism into failure is to provide limit switches to stop the motor when a terminus point has been reached. Limit switches, however, suffer from low reliability, they add complexity and cost to the system, and they undesirably increase the overall size of the transfer switch. Another solution is to, attach an external sensor, such as a tachometer or an optical or magnetic encoder, to the motor shaft that provides data indicating the position of the motor. Like the limit switches, external sensors add complexity, cost, and size to the system.
In addition, a transfer switch must assure that its load is connected to only one of the sources at a time. Exclusively connecting to just one source or the other is usually implemented using a mechanical interlock. A transfer switch is composed of one switch per source that when closed connects the load to that source. The interlock mechanically permits only one switch to be actuated to a closed position at a time.
However, the actuating mechanism of a switch, such as a handle, cannot always be guaranteed to close or open the electrical contacts of the switch. If the handle, or other critical internal part, were to break, the switch could be electrically closed although the switch has been actuated to an open position.
In the case of the transfer switch, such breakage would cause the mechanical interlock to fail allowing multiple sources to be connected to the load, causing two asynchronous power sources to short together thereby potentially damaging both. A breakage could also cause a powered source to reverse power an un-powered source. If a backup generator were running and connected to an un-powered utility line, the utility line would become back-fed and pose a serious safety hazard to the utility lineman.
Thus, a need exists for an improved apparatus and method. The present invention is directed to satisfying one or more of these needs and solving other problems.